PROJECT SUMMARY Rotator cuff tears affect about 40% of the population over age 60 and are a common cause of pain and disability. Approximately 250,000 rotator cuff repairs are performed in the United States each year, but healing following surgery is a significant challenge (e.g., 20-70% of surgical repairs fail) and post- operative shoulder function is unpredictable. There is also often a disconnect between repair tissue healing and shoulder function where patients have poor shoulder function (e.g., limited strength and pain) despite an intact repair or, conversely, excellent shoulder function despite a failed repair. Conventional clinical data (e.g., patient age, tear size) are not strong predictors of clinical outcome, and therefore this disconnect between healing and function remains difficult to explain. Recent research suggests that repair tension and repair tissue elongation may provide insight into post-operative healing and shoulder function that is not adequately provided by clinical data. However, the relationships between repair tension, repair tissue deformation, healing, and shoulder function are not well understood. The objectives of this application are to determine how rotator cuff repair affects shoulder motion, strength, and patient-reported outcomes, and to assess the influence of repair tension and repair tissue deformation on these outcomes. The rationale for this project is based on several important findings from our on-going work regarding the progression and treatment of rotator cuff tears: 1) rotator cuff pathology, even in the absence of symptoms, has a significant impact on shoulder function, 2) physical therapy improves clinical outcomes despite only minor changes in joint motion, 3) surgical repair appears to alter glenohumeral joint (GHJ) motion in a way that suggests excessive repair tension, and 4) shoulder motion, strength, and patient-reported pain/function scores are inter- related after surgery. Based on these findings and the purported roles of repair tension and repair tissue elongation, our central hypothesis is that repair tissue elongation (up to and including failure) is due, at least in part, to repair tension approaching or exceeding the mechanical capacity of the healing repair tissue. We also hypothesize that repair tissue deformation affects joint motion in ways that have a significant impact on strength and patient-reported outcomes. Our approach will be to conduct a longitudinal study that measures repair tension, repair tissue deformation, joint motion, strength, and patient-reported outcomes before and after surgical repair. The proposed research is innovative because it will use a state-of-the-art imaging technique to provide an accurate assessment of the mechanical progression of healing rotator cuff repair tissues. The contribution of this research will be significant because it will advance our understanding of how surgical repair influences shoulder function and clinical outcomes, ultimately leading to improved patient care.